1. Field of the Invention
The present invention relates to the field of chromatic separation, where a single light beam that is composed of many colors is separated into individual beams of different colors. More specifically, the invention relates to diffraction gratings used as chromatic separation devices.
2. Description of Related Art
The amount of data that can be transferred using an optical fiber is based on the bandwidth that the fiber can support. This value can be very large for modern fibers, leading to a theoretical limit of  greater than 10 Tbits/s for a single fiber cable link. However, in practice this bandwidth cannot be fully utilized, primarily due to electronic limitations in the generation and reception of laser beams modulated at this very fast rate. Wavelength division multiplexing (WDM) is a way to utilize more of the available bandwidth in an optical fiber by sending multiple individual channels, with different wavelengths, down a single fiber. The achievable data rate is now dependent on the number of channels multiplied by the data rate in each channel, which contains data at a rate able to be processed by modern electronics. WDM and DWDM systems have been deployed extensively for long-haul telecommunications links and are now being installed in shorter-range links such as metropolitan areas.
One of the key components for DWDM systems is a device which allows multiple beams of light, each having a slightly different wavelength corresponding to a communication channel, which are co-propagating in a single-mode optical fiber, to be separated spatially so that the information in each channel can be processed. Also, the reverse of this is needed, whereby individual channels having different wavelengths are recombined from individual fiber optic cables into a single cable. Gratings are commonly used to perform this spatial separation, by acting as chromatic dispersion elements. Existing DWDM components couple light out of fiber optic cable and back into different cable(s) a set distance away, striking a diffraction grating in the middle in order to provide separation or combination of the different colors that make up the data channels.
U.S. Pat. No. 4,111,524, titled xe2x80x9cWavelength Division Multiplexerxe2x80x9d, issued Sep. 5, 1978, teaches the use of a diffraction grating within a low loss multiplexing system. There are issues with using diffraction gratings for this purpose in that light passing through standard fiber optic cable becomes randomly polarized after traveling relatively short distances through the fiber. Unpolarized light is a problem for diffraction gratings with a pitch that is close to the wavelength of light, since the efficiency of the polarization states cannot be matched without significant losses using standard lamellar or sinusoidal single-layer groove structures. A grating with a pitch that is similar to the wavelength size allows the maximum dispersion of light incident on it, and so is desirable for the separation of high channel density DWDM.
The desire to increase the transferred data rate of DWDM systems has caused an increase in the total bandwidth that the channels take up. This is also a problem for grating-based demultiplexers/multiplexers, since the grating is required to have high performance (low loss) over a wide wavelength range. Conventional gratings typically have a small bandwidth region of optimum performance.
There is prior art concerned with finding methods to control polarization losses in grating based systems. One example is U.S. Pat. No. 6,097,863, titled xe2x80x9cDiffraction Grating with Reduced Polarization Sensitivityxe2x80x9d issued Aug. 1, 2000. The desire for improvement in gratings is not limited to telecommunications, several inventors have produced relevant advances in gratings for use in other fields, e.g., high-power lasers, such as U.S. Pat. No. 4,313,648, titled xe2x80x9cPatterned Multi-Layer Structure And Manufacturing Methodxe2x80x9d and U.S. Pat. No. 5,907,436, titled xe2x80x9cMultilayer Dielectric Diffraction Gratingsxe2x80x9d which involve the use of dielectric multilayer grating structures.
This invention has applications in spectroscopy, short-pulse lasers and optical telecom, among others. The invention details a new type of diffraction grating that has improved properties for chromatic dispersion, notably for dealing with a system with large chromatic dispersion involving unpolarized light.
The chromatic dispersion properties of a grating can be used to separate individual channels of a DWDM system by causing each channel to be diffracted at a different angle. This process causes a spatial separation of the channels at a later image plane, so that each can be coupled into a different fiber optic. These systems can also be used in reverse, where the light from individual channels are combined into one fiber by using the chromatic dispersion properties of a grating to re-direct each channel by a predetermined amount. Gratings have many advantages over other methods in that since they use an angular separation, they can be directly scaled for increasing channel number and decreasing channel spacing systems by changing the position of the output plane.
However the light for each channel in a DWDM system is not polarized. Conventional gratings with a wavelength-scale period are normally very polarization sensitive, having very different diffraction efficiencies for different polarizations. Their use requires a trade-off between total loss and polarization dependent loss of the system, i.e., the total diffraction efficiency is normally reduced for a solution in which the efficiency of each polarization of the incoming light is matched. Also, diffraction gratings for modern telecommunications are required to operate on many channels over a xcx9c35 nm bandwidth with uniform performance. This is also an issue for conventional gratings, since performance is typically a strong function of bandwidth, and like the polarization case, the overall performance is diminished.
Dielectric gratings, with a period of similar size to the wavelength of the incident light, can be designed so that light is diffracted with near 100% efficiency for a specific wavelength and polarization. However, these types of gratings cannot produce this performance for an unpolarized beam over a range of incident wavelengths and angles. Larger-period gratings, such as mechanically ruled blazed gratings, operate in a totally different diffraction regime, and so are polarization independent devices. Also, the reflective properties of the metallic surface gives them high diffraction efficiency for a large wavelength range. However, the smaller the grating period, the higher the angular separation for each channel, so larger-period gratings are not as effective for channel separation, leading to larger, less stable, devices.
Using a hybrid structure, consisting of a base metallic reflecting layer with a diffraction grating on its surface that is formed from multilayer dielectric stacks, allows for the polarization insensitive reflective properties of the base metallic layer to operate in conjunction with the polarization sensitive diffraction properties of the multilayer grating structure so that a grating can be produced that has near 100% diffraction efficiency over a reasonable wavelength bandwidth, independent of the polarization of the incident beam.